This invention relates to borehole telemetry systems for transmitting indications of subsurface conditions to the earth's surface and, more particularly, to a borehole telemetry system using explosive seismic sources to transmit signals to the earth's surface.
Numerous devices have been developed for the detection of subsurface conditions such as temperature and pressure in a borehole. An example is U.S. Pat. No. 3,905,010 issued to John Fitzpatrick on Sept. 9, 1975. The device disclosed by this patent includes pressure and temperature transducers which generate electrical outputs representative of these two formation parameters. These outputs are converted to electromagnetic radiation in the microwave range which is then transmitted up a tubing string to a receiver at the surface. Also disclosed in the patent is the use of fiber optics and modulated laser beams to transmit the same energy to the surface.
Other downhole sensing systems are known in which acoustic waves are used to transmit energy to the earth's surface. Most typically, these involve an acoustic transmitter which generates waves in the drilling fluid, which then travel in the fluid in a casing string to the earth's surface. Other devices are known which generate acoustic waves in the well casing, which then travel up the well casing to the earth's surface to be detected there. Variable frequency mud sirens have been used in logging while drilling operations which transmit energy by means of acoustic waves traveling through the drilling fluid in the drill string.
Numerous sensors are also known such as that disclosed in U.S. Pat. No. 2,718,145 issued to Nisle on Sept. 20, 1955. This transducer generates an electrical resistance change proportional to pressure being detected and must be directly connected by means of wires to either a telemetering or recording device.
Exploratory wells are sometimes drilled solely for the purpose of obtaining core samples and logging measurements of the subsurface formations. Quite often, wells drilled for the purpose of production of oil and gas turn out to be dry holes and thus are useful only for the information they can yield toward a better understanding of the formations in that area. Subsurface temperature and pressure are two of the parameters which aid geologists in understanding the subsurface conditions. Both of these parameters are changed by the process of drilling a borehole, and measurement of them at that time or shortly thereafter is not as valuable as a measurement after the temperatures and pressures have returned to their original states. Many of the known devices for measuring such parameters, such as those mentioned above, depend upon the presence of casing or drill strings to transmit the signals back to the surface. These expensive materials are not normally left in exploratory or dry holes.
While sensors coupled to the surface by means of electrical conductors can be placed in uncased boreholes, there are several problems. It is difficult to place a sensing device at the bottom of the borehole with wires connecting it to the surface without damaging those wires in the process of placing the device in the hole. Such boreholes must be plugged with concrete before they are abandoned. The cementing operation also requires operations within the hole which can damage wires. In addition, most state drilling regulations do not consider a well to be properly plugged if wires are left in the borehole so that they pass through the plug.
It is conceivable that some of the known acoustic generating devices could be used at the bottom of a plugged, abandoned borehole; but, without the presence of casing or drill string to act as a conduit for the acoustic energy, the energy output of such devices would have to be greatly increased to be detectable at the earth's surface. With present devices, the energy for such transmissions would have to be stored in batteries within the device. Since pressure and temperature conditions vary slowly and over long time intervals, it is desirable to detect such signals and transmit them to the surface at intervals in the range of weeks or months. At the present time, it is not practical to provide batteries with sufficient stored energy to generate the required level of acoustic energy even for a short period of time, much less repetitively over a long period of time.
Accordingly, an object of the present invention is to provide an improved downhole sensing and telemetering device capable of transmitting acoustic waves indicative of downhole conditions at time intervals long after the borehole is plugged.
Another object of the present invention is to provide a relatively inexpensive and simple downhole sensing and telemetering device which employs explosive charges as a source of transmitted acoustic energy.
A downhole sensing and telemetering device, according to the present invention, comprises a sensor such as a temperature sensor, a timer, and a series of individual explosive charges. The sensor provides an electrical output which is a function of the parameter, such as temperature, being detected. The timer provides at least two time intervals, the first time interval being a delay time such as one week at which information will be transmitted, and the second time interval being proportional to the parameter being transmitted. A first of the charges is detonated at the end of the delay time interval and a second at the end of the second time interval.